I'm pretty sure it is a legal and company issue.
Legal because I don't think you can straight port anything that was on another console because they own some rights to that game, I think, I'm just guessing really.
Company because, face it, what company really wants to spend the time and money dedicating an entire team to just rehash old games for the newer consoles when they can make, you know, new games instead. Plus, how much money could they really get through this tactic. Plus, how old are we talking about here? Are we thinking of old NES games because again, it would seem like a waste of time for developers seeing as everyone and there mom knows how to get a ROM for those on a PC.
I would rather see maybe just as far back as PS1,Sega Saturn, Dreamcast . I know XBOX Live has xbox originals and other games such as Castlevania: Symphony of the Night and i got some old SEGA roms. Since there are no more games being developed for these consoles, i wouldnt mind playing some of these games. I had fun play jet set radio and space channel 5 for the dreamcast.im pretty sure with the way games are developed now rehashing old discs wont be so hard and time consuming.